Prior-art measuring devices for determining and/or monitoring an arbitrary process variable (e.g., level, pressure, temperature, flow rate) in industrial measurement technology are so designed that only the respective current measured value is stored and made available. For diagnostic, error-detection, and predictive-maintenance purposes, however, the provision of the current measured value is very seldom sufficient. For diagnosis and for the detection of incipient faults, it is necessary to record measurement and/or system information over a prolonged period of time, so that it can be retrieved and evaluated when required.
So far it has only become known to connect a so-called data logger to the measuring device proper in case of need. The data logger is capable of recording the measurement data over a desired period of time for future reference. Commonly used data loggers are offered and sold by Endress+Hauser Wetzer GmbH+Co. KG, for example. These data loggers are used, for instance, when a malfunction of the device is presumed. Using the measurement and/or system data recorded by the data logger over time, a fault diagnosis can be carried out. However, before the fault is actually located, much time may pass.
Error detection is particularly difficult if a malfunction occurs only intermittently, for instance at irregular intervals. In that case it may happen that in the current measuring period, in which the data logger is recording measurement and/or system data, the malfunction is not detectable. The measuring device then operates error-free during the recording of the measurement data; it may happen, however, that the malfunction recurs in the near future and, in the extreme case, is not detectable by a second check via the data logger, either.
Thus, the known analysis with the help of a data logger which is connected to the measuring system only from time to time involves the risk that despite a great number of checks, an intermittently occurring malfunction of the measuring device cannot be detected and, consequently, not be corrected. The method employed in the prior art is therefore uneconomical and may even be dangerous in critical applications. An example of a critical application is an intermittently occurring fault in an overfill safeguard mounted in a tank which holds chemicals that are injurious to health.